Auto-monitoring is a requirement imposed by Underwriter Laboratory Standard UL-943 for ground fault circuit interrupter (GFCI) devices in the United States. These GFCI devices are commonly used in both commercial and residential applications and include receptacles, circuit breakers, plug-in devices, and the like. The auto-monitoring requirement arose from a realization by regulators that consumers do not regularly test their GFCI devices (e.g., by pushing the “Test” button). As a result, many GFCI devices that were no longer operational due for example to aging remained in use rather than being replaced, creating potential electrical safety hazards.
UL-943 addresses the above problem, among others, by requiring a permanently connected GFCI device to have an auto-monitoring function that periodically and automatically tests the ability of the device to respond to a ground fault. If the device fails the test, the standard requires the device to enter automatically into a state that prevents the device from energizing the load, either by blocking power to the load or by repeatedly tripping. The standard additionally specifies that the auto-monitor testing must not compromise the ability of the GFCI device to respond normally to the ground fault or a grounded neutral fault.
One of the ways specified by the standard for confirming compliance with the auto-monitoring requirement is to open-circuit the trip solenoid and the switching semiconductor in the device. However, in electronic circuit breakers, the trip solenoid and associated switching semiconductor operate to actuate the trip mechanism that cuts off electricity to the load. Any test that requires the trip solenoid or the switching semiconductor to be open-circuited would render them unable to actuate the trip mechanism. Consequently, an exception was specified in UL-943 that allowed circuit breakers to avoid the auto-monitoring testing of the trip solenoid and the switching semiconductor.
But trip solenoids and switching semiconductors are susceptible to failure like other electrical components. Not testing them may lead consumers to believe incorrectly that non-operational circuit breakers are still functioning properly, there being no indications to the contrary. This creates potential electrical safety hazards. Moreover, a request was submitted recently to the UL urging removal of the circuit breaker trip solenoid and switching semiconductor exception. If the UL adopts the request, which seems likely, circuit breakers will need to comply fully with the auto-monitoring testing.
Prior attempts to implement auto-monitoring in circuit breakers have involved locking out the circuit breaker after, not during, a test. For example, US Published Application No. 20080186112 describes a circuit breaker that does not trip in response to a failed push-to-test, but once turned off, the circuit breaker is locked out and cannot turn back on. However, these attempts still require a manual operation by the user, potentially permitting unsafe circuit breakers to continue delivering power. They also require additional electronic components and mechanical interfaces, which is not feasible in certain circuit breaker applications, such as MCBs, that have strict size constraints.
Accordingly, a need exists for a way to fully accommodate the auto-monitoring requirement in circuit breakers, even in circuit breakers with strict size constraints, such as MCBs.